This invention relates to the manufacture of printed circuit boards, and in particular, to an apparatus and method for conveying boards that have a circuit pattern formed thereon through a circulating air dryer or finishing oven.
During the manufacture of printed circuit boards a resist material is applied to the board using a magnetic printing process. In this process a circuit pattern of magnetized toner is transferred to the board from a printing cylinder the surface of which has been magnetized in the pattern of the circuit. The soft, sensitive toner image on the board is then annealed by heat treatment before further processing of the board may take place. This process is described in more detail in U.S. Pat. No. 4,338,391 (Nacci et al.).
Until now infrared radiation has been used for heat treating such circuit boards that are printed on one or both sides. However this has entailed a risk of damaging (e.g. permanently distorting or buckling) the board's substrate because of the high energy density required. In particular, a thin circuit board and, even more particularly, a thin board coated on one side, tends to sag in the direction of the heat source due to the bimetallic effect. In addition, since the radiant energy varies inversely as the square of the separating distance the continuous radiation process involves unusual problems due to nonuniform distribution of temperatures between the center and the outer areas of the circuit board. Since the circuit board is usually printed out to the edge, and since the printed surfaces must not be touched until they are completely hardened, heat treatment by infrared radiation involves significant problems in conveying the board through the treatment unit. Still further, serious problems existing in connection with conveying the circuit board have so far prevented the use of uniformly heating and gently working circulating air ovens.
It is therefore believed advantageous to provide an apparatus and method in which a flat planar member, such as a printed circuit board, is conveyed through a circulating air dryer or finishing oven. It is believed to be of further advantage to hold the planar member during its treatment in a receptor device or basket without the risk of the surface of the member being touched by the basket. It is believed to be of yet further advantage to permit the planar member to move within the basket in which it is carried to position itself properly for the treatment being conducted on it without risking the danger of buckling.